Doom (officially cased DOOM) is a 1993 computer game by id Software that is a landmark title in the first-person shooter genre, and in first person gaming in general. It is widely recognized for pioneering immersive 3D graphics, networked multiplayer gaming on the PC platform, and support for custom expansions (WADs). Distributed as shareware, Doom was downloaded by an estimated 10 million people within two years, popularizing the mode of gameplay and spawning a gaming subculture; as a sign of its effect on the industry, games from the mid-1990s boom of first-person shooters are often known simply as "Doom clones". Its graphic and interactive violence has also made Doom the subject of much controversy reaching outside the gaming world. According to GameSpy, Doom was voted by industry insiders to be the greatest game of all time in 2004.

The Doom franchise was continued with Doom II: Hell on Earth (1994) and numerous expansion packs, including The Ultimate Doom (1995), Master Levels for Doom II (1995), and Final Doom (1996). Originally released for PC/DOS, these games have later been ported to many other platforms, including nine different game consoles, Rockbox firmware, and even PDAs and the Flash Player virtual machine. The series lost mainstream appeal as the technology of the Doom game engine was surpassed in the mid-1990s, although fans have continued making WADs, speedruns, and modifications to the source code released in 1997. The franchise again received popular attention in 2004 with the release of Doom 3, a retelling of the original game using new technology, and an associated 2005 Doom motion picture.

Story

Doom has a science fiction/horror theme, and a simple plot. A background story is given in the game's manual, and the rest of the story is advanced with short messages displayed between each section of the game (called episode), the action as the player character progresses through the levels, and some visual cues.

The player takes the role of a space marine, "one of Earth's toughest, hardened in combat and trained for action", who has been deported to Mars as punishment for assaulting a senior officer when ordered to attack civilians. He forms part of a force stationed in the area where the Union Aerospace Corporation (UAC), a multi-planetary conglomerate, is performing secret experiments with teleportation between the moons of Mars, Phobos and Deimos. The marine seems to have been forced into a security or unimportant staff position according to the manual, stating that "with no action for fifty million miles, your day consisted of suckin' dust and watchin' restricted flicks in the rec room." Suddenly, computer systems on Phobos behave wildly and unpredictably, and "something friggin' evil" comes out of the teleporters, called "gateways". An emphatic call for assistance is made to Mars as the bases get overrun, but all personnel are killed or possessed. At the same time, Deimos vanishes entirely. The military unit is sent to Phobos to investigate the incident, and the player character is left to guard the hangar as the rest of the group proceeds. Radio signals filled with gunfire and other signs of violence indicate the group engages the invaders, and eventually it becomes apparent that the marine is all alone on hostile Phobos.

In order for the game to be completed, the marine must fight through Phobos, Deimos, and then Hell, each presented as an episode containing nine distinct levels. Knee-Deep in the Dead, the first episode and the only one in the shareware version, is set in the high-tech military bases on Phobos. It ends with the player character fighting a pair of Barons of Hell and afterwards entering the teleporter leading to Deimos, ending with him getting overwhelmed by monsters, if not killed. In the second episode, the Shores of Hell, the character journeys through the installations on Deimos, whose areas are interwoven with beastly architecture, warped and distorted by the hellish invasion. After defeating the titanic Cyberdemon lord, he discovers the truth about the vanished moon: it is floating above Hell. After climbing down to the surface, the third episode begins, called Inferno. After the huge Spiderdemon that masterminded the invasion is destroyed a hidden doorway back to Earth opens for the hero, who has "proven too tough for Hell to contain".